Life Around Dying Stars

by Paul Gilster on February 26, 2013

Where is the best place to look for life? At first glance, a red dwarf would seem to be the ideal choice because a transiting terrestrial-class world in the habitable zone of a red dwarf is going to block a larger part of the star’s light than a similarly sized world orbiting a larger star. Red dwarfs pose their own problems for life, including the possibility of tidal lock and severe flares, but in terms of detectability, they seem made to order for planet hunters with transit methods in mind.

But white dwarfs turn out to be interesting targets in their own right, and in at least one significant way may offer even more advantages. So says a new paper by Avi Loeb (Harvard-Smithsonian Center for Astrophysics) and Don Maoz (Tel Aviv University), who point out that a habitable planet orbiting a white dwarf would have to be close to its star indeed, perhaps as close as 1.5 million kilometers. As with a red dwarf, a transit here will block a large fraction of the star’s light — astronomers speak of a large ‘transit depth’ — and should therefore be readily detectable.

But there is more to the story. Loeb and Maoz are interested in not just detecting such a world but studying its atmosphere. We’ve already been able to examine the atmosphere of the gas giant HD 209458 using Hubble data, so the method is known to work. Astronomers make spectroscopic observations of the unobstructed light of the star and compare these to measurements of the star during a planetary transit. It’s tricky work, and getting it down to the scale of smaller planets will require future instruments like the James Webb Space Telescope.

JWST, in fact, should be able to make atmospheric measurements down to planets of just a few Earth masses in the habitable zone of red dwarfs, but the paper points out how many hours of total exposure time will be necessary and how many conditions will need to be right for this to happen. But Loeb and Maoz argue that the signature of water vapor and perhaps oxygen will be detectable by the Webb instrument in just a few hours if found on a planet around a white dwarf. The researchers simulated a JWST observation of an Earth-like planet’s transit across such a star and found that the atmospheric ‘transmission signal’ is much more detectable than around any main-sequence stars, with oxygen in particular being a feasible catch for the JWST.

Image: A new study finds that we could detect oxygen in the atmosphere of a habitable planet orbiting a white dwarf (as shown in this artist’s illustration) much more easily than for an Earth-like planet orbiting a Sun-like star. Here the ghostly blue ring is a planetary nebula – hydrogen gas the star ejected as it evolved from a red giant to a white dwarf. Credit: David A. Aguilar (CfA).

But can planets form around white dwarfs, which are, after all, the remnants of red giants? Recent work has shown that stars like these can have long-lasting habitable zones, and studies of the photospheric metals in these stars have led to estimates that between 15 and 30 percent could host planets. A small planet within a few AU of its star would not survive the red giant phase, so we are talking about planets that migrated in from a much wider orbit after the white dwarf had formed. And even assuming such, we still have issues of water loss:

Barnes & Heller (2012) and Nordhaus & Spiegel (2012) have emphasized that the tidal heating of the planet, until it had achieved full circularization and synchronization, would lead to full loss of any water and volatiles present. We note, however, that the young Earth was also a hot and dry place, but volatiles and water were then delivered to it by a barrage of comets. The comet impact rate then decreased to its present low level, greatly lowering the biological damage of such impacts. It is not implausible that such post-formation volatile delivery also could take place on an earth-like planet in a WD’s habitable zone, perhaps driven by the same scattering process that drove the planet itself to migrate inward after the formation of the WD.

All this leaves us with the question of finding white dwarfs with transits to study. The authors call for sampling some 500 white dwarfs within approximately 40 parsecs, a study that should be feasible with the European Space Agency’s Gaia observatory, scheduled for launch later this year. These targets would then be monitored with small telescopes in search of transits. “Earth-mass planets in the habitable zones of WDs,” write the authors, “may oﬀer the best prospects for detecting bio-signatures within the coming decade.”

The paper is Loeb and Maoz, “Detecting bio-markers in habitable-zone earths transiting white dwarfs,” accepted for publication in Monthly Notices of the Royal Astronomical Society (preprint).

Hm, I am sorry but I believe in white dwarfs as harbors of life even less than I believe in red dwarfs, in fact much less so.
As mentioned in the article a prerequisite is that the planet must either have migrated inward after the star became WD, or have formed anew from debris.
Such WDs must have an extremely narrow HZ, so the chance of an earthlike planet migrating or forming exactly in it must also be tiny.
And talking about tidal locking!
I think this is an extreme case of wishful thinking.
Let’s face it, a WD is the carcass of a star.

An extreme long-shot, for many reasons, just one of which is that the WDs mostly are substantially older than our Sun, implying that their planets likely are deficient in the “nourishing” chemical elements only available when stellar nucleosynthesis after they were born would have further enriched the galaxy’s carbon, nitrogen, carbon, etc. On the other hand, if an advanced civilization had traveled and settled there after the planet migrated inward . . . .

The astro-engineering possibilities are worth exploring. For example, in Olaf Stapledon’s “Last and First Men”, there was talk of moving the planets inwards as the Sun was expected to cool after the catastropic brightening that forced the migration to Neptune. If a very efficient neutrino reaction and rocket could be developed, using some kind of inverse baryogenesis, then Earth (and other planets) could be sent in-spiralling towards the Sun after it begins its white dwarf phase.

According to Martin Beech’s astro-engineering work the Sun could have its useful lifespan extended many-fold by siphoning off “excess” mass. A necessity of the “easiest” scenario involves shifting the planets outwards as its luminosity increases slowly. Then, once a helium core develops, the Sun can be allowed to become a helium white-dwarf and the planets can spiral back inwards. The excess mass could be used to make low-mass companion stars, eventually creating a quintet of red-dwarfs. The various terraformed planets can be shared out between the new low-mass stars. Depending on the exact parameters chosen, the Earth could end up orbiting a quasi-terraformed gas-giant around one of the new stars.

How does one terraform Jupiter or Saturn, you might wonder. That’s a whole other story…

There are an awful lot of implicit ‘ifs’ in that paper, but if we leave aside the incredibly unlikely astrobiological side of things, just detecting some transits around white dwarfs would be fascinating! The pollution studies suggest there’s something going on. Sure, they’re probably going to be airless boulders, but still – planets, around white dwarfs!

I’m not sure the narrowness of the HZ in absolute terms is in itself too much of an issue: planetary systems get more compact as you go inwards (or to put it another way, would we really expect the probability of finding a planet being between 1 and 2 AU to be the same as that between 51 and 52 AU?).

Issue is the initial state of the white dwarf is extremely nasty: it has an extremely high temperature and is very luminous. Is the eventual HZ within the dust sublimation radius?

Maybe you could get around the hot young white dwarf problem in a binary system: take a white dwarf with a companion star, and perhaps the white dwarf can capture an accretion disc from the material ejected as its companion goes through its own red giant stage which could then produce its own set of planets.

Would be interesting to see if the Kepler Telescope records any transits across white dwarf starts in its field of view.

What about life around younger, massive stars? A recent article on a planet detected around HD 100546 (http://www.sciencedaily.com/releases/2013/02/130228103341.htm) raises the plausibility of planets (possibly habitable?) orbiting B and A main sequence stars. HD 100546 reportedly hosts two planets at 6 AU and 70 AU, suggesting more massive stars could support vast solar systems, with wider habitable zones compared with smaller stars. Perhaps Kepler will get lucky and spot planetary transits across more massive stars. Granted, B and A-class stars have shorther lifetimes and hence any habitable planets may not have much in the way of complex life, or even an oxygen atmosphere. And they’ll be pummeled with debris orbiting these younger stars. But it’d make my day to read about the discovery of a Rigel VII or a Sirius X!

As the lighter elements get boiled off, maybe life with odd biochemistries might become more possible.
Wikipedia’s article on hypothetical types of biochemistries suggests some really odd ones, like titanium-based life.

As a planet with a mass of 10 times Jupiter’s migrates inward, its mass boils off and it might end up with the same mass as Earth or Mars, and develop life with some odd biochemistries.

About 1.1 billion years from now, the sun will begin to change. As the hydrogen fuel in its core is used up, the burning will spread outward toward the surface. This will make the sun grow brighter. This increased radiation will have a devastating effect on our planet. Here’s what that might look like.

Because it has no source of energy, a dead star — known as a white dwarf — will eventually cool down and fade away. But circumstantial evidence suggests that white dwarfs can still support habitable planets, says Prof. Dan Maoz of Tel Aviv University’s School of Physics and Astronomy.

Now Prof. Maoz and Prof. Avi Loeb, Director of Harvard University’s Institute for Theory and Computation and a Sackler Professor by Special Appointment at TAU, have shown that, using advanced technology to become available within the next decade, it should be possible to detect biomarkers surrounding these planets — including oxygen and methane — that indicate the presence of life.

Published in the Monthly Notices of the Royal Astronomical Society [preprint: http://arxiv.org/abs/1301.4994, the researchers’ “simulated spectrum” demonstrates that the James Webb Space Telescope (JWST), set to be launched by NASA in 2018, will be capable of detecting oxygen and water in the atmosphere of an Earth-like planet orbiting a white dwarf after only a few hours of observation time — much more easily than for an Earth-like planet orbiting a Sun-like star.

Their collaboration is made possible by the Harvard TAU Astronomy Initiative, recently endowed by Dr. Raymond and Beverly Sackler.

Faint Light, Clear Signals

“In the quest for extraterrestrial biological signatures, the first stars we study should be white dwarfs,” said Prof. Loeb. Prof. Maoz agrees, noting that if “all the conditions are right, we’ll be able to detect signs of life” on planets orbiting white dwarf stars using the much-anticipated JWST.

An abundance of heavy elements already observed on the surface of white dwarfs suggest rocky planets orbit a significant fraction of them. The researchers estimate that a survey of 500 of the closest white dwarfs could spot one or more habitable planets.

The unique characteristics of white dwarfs could make these planets easier to spot than planets orbiting normal stars, the researchers have shown. Their atmospheres can be detected and analyzed when a star dims as an orbiting planet crosses in front of it. As the background starlight shines through the planet’s atmosphere, elements in the atmosphere will absorb some of the starlight, leaving chemical clues of their presence — clues that can then be detected from the JWST.

When an Earth-like planet orbits a normal star, “the difficulty lies in the extreme faintness of the signal, which is hidden in the glare of the ‘parent’ star,” Prof. Maoz says. “The novelty of our idea is that, if the parent star is a white dwarf, whose size is comparable to that of an Earth-sized planet, that glare is greatly reduced, and we can now realistically contemplate seeing the oxygen biomarker.”

In order to estimate the kind of data that the JWST will be able to see, the researchers created a “synthetic spectrum,” which replicates that of an inhabited planet similar to Earth orbiting a white dwarf. They demonstrated that the telescope should be able to pick up signs of oxygen and water, if they exist on the planet.

A Critical Sign of Life

The presence of oxygen biomarkers would be the most critical signal of the presence of life on extraterrestrial planets. Earth’s atmosphere, for example, is 21 percent oxygen, and this is entirely produced by our planet’s plant life as a result of photosynthesis. Without the existence of plants, an atmosphere would be entirely devoid of oxygen.

The JWST will be ideal for hunting out signs of life on extraterrestrial planets because it is designed to look into the infrared region of the light spectrum, where such biomarkers are prominent. In addition, as a space-based telescope, it will be able to analyze the atmospheres of Earth-like planets outside our solar system without weeding out the similar signatures of Earth’s own atmosphere.

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In Centauri Dreams, Paul Gilster looks at peer-reviewed research on deep space exploration, with an eye toward interstellar possibilities. For the last nine years, this site has coordinated its efforts with the Tau Zero Foundation, and now serves as the Foundation's news forum. In the logo above, the leftmost star is Alpha Centauri, a triple system closer than any other star, and a primary target for early interstellar probes. To its right is Beta Centauri (not a part of the Alpha Centauri system), with Beta, Gamma, Delta and Epsilon Crucis, stars in the Southern Cross, visible at the far right (image: Marco Lorenzi).

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